Livestock are more likely to eat poisonous plants when pasture grass is overgrazed

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Pastures that have been seriously overgrazed or overstocked can predispose livestock to eating anything they can, especially if they are hungry, according to a clinical science professor at Colorado State University. Veterinarian Dr. Tony Knight spoke to a group of over 60 people about poisonous plants and their risk to livestock during a recent presentation.

Several factors can lead to livestock becoming poisoned from eating toxic plants. Producers should evaluate their pastures to determine if adequate forage exists before turning livestock out to graze. If there is a lack of adequate forage, livestock may be more likely to eat poisonous plants, Knight said. Some poisonous plants can be more harmful depending upon the season of the year, Knight continued. For instance, Larkspur is the most toxic in the spring during the pre-flowering stage.

Knight said some poisonous plants can also be more toxic if they are affected by drought. Animals that are brought from a different state may also be more likely to taste a poisonous plant if that plant isn't something they're familiar with. Producers also need to observe vegetation in a pasture when it is covered with snow to make sure the animals can still reach basic forages. "Sometimes, the snow will cover up basic forage vegetation, leaving poisonous plants exposed for grazing," he explained.

Since animals tend to be choosy in what they eat, Knight said, "if there is plenty of forage to eat, animals will usually avoid poisonous plants." He also informed the group that some plants can be poisonous throughout the year, even when they are dry or put up into hay.

"If an animal eats a mouth or two of a toxic plant, they rarely get enough to cause poisoning," Knight said. "It is the dose that makes the toxin." Each animal will also respond differently, he continued. "Sheep and goats are more adaptable to eating poisonous plants. That is why they are used as biological controls for some poisonous plants," he added.

"Cattle probably eat more poisonous plants than any other animal," the veterinarian said. "They are voracious eaters consuming a larger amount of plant material into their rumen. The dose is the factor that will determine if they are poisoned," he explained. If a mob of animals are turned onto a piece of grass with poisonous plants, each animal will consume less of the poisonous plants because of the number of animals.

"Toxins can vary depending upon the stage of growth of the plant," Knight explained. The level of toxicity can also be dependent on if the plant is hydrated or droughted, if it is growing in the shade or in the sun, the animal species grazing it, and the nutritional status of the animal. Knight also cautioned against spraying some poisonous plants. "In some cases, herbicide spraying can actually increase the palatability of the plant, and increase the nitrate levels making the animal more likely to consume the plant," he said.

Knight discussed some of the most poisonous native plants in this area, which are Locoweed, Larkspur, Veratrum, Death Camas, Sage, Ponderosa, Snakeweed, and Water Hemlock. "These native plants are more poisonous than noxious weeds that are invasive," he said.

Larkspur

Over 60 species of Larkspur exist in North America, Knight said. "Larkspur contains complex alkaloids that work on the nervous system to cause paralysis and rumen bloat." Symptoms of Larkspur poisoning include muscle weakness, staggering gait, respiratory difficulty, bloating and unable to belch, and eventually death if the animal isn't quickly treated by a veterinarian.

The affected animal can be treated with Physostigmine, if it is given intravenously or intramuscularly before the animal collapses. If it is given in time, it will reverse paralysis. The veterinarian may also need to treat the bloat and reduce stress on the animal, he added.

Cattle are most affected by consuming Larkspur. Horses can be somewhat affected, and sheep can be used as a biological control, Knight said. "If I had neighbors with lots of goats and sheep, I would talk with them about grazing the pasture as a biological control of Larkspur," he said.

Larkspur is the most toxic in the pre-flowering stage," he continued. "The key is to identify the plant before the flowering stage, and remove the animals from that area until the plants reach the seed pod stage when the plant is less poisonous," he said.

Water Hemlock

One of the most poisonous native plants is Water Hemlock, according to Knight. The highly toxic plant grows in marshy ground, and is identifiable by its white flowers. If the plant is pulled out of the ground, it has a hollow stem and tuberous, finger-like roots. If the roots are cut, a yellow-like fluid leaks out that is highly toxic.

"Water Hemlock is poisonous to all animals and humans," Knight cautioned. "It only takes a few ounces to kill a 1,000 pound animal. There is no antidote," he stated. "This is one plant I would recommend you remove if you find it."

Knight added the poison or spotted hemlock can grow up to six foot tall, and is a prolific seed producer. "I would recommend removing it before it goes to seed to keep it from spreading," he said.

Spotted hemlock has carrot-like leaves with red spots on the stems. It can cause sudden death in animals, and fetal abnormalities in cows that browse on it while they are pregnant.

Milkweed

Milkweed is identified by its pink or white flowers that the Monarch butterfly likes to lay its eggs upon. According to Knight, if an animal ingests one to two pounds of its leaves, the animal could die. "It is the most poisonous when green, but poisoning can still occur in the fall when the leaves dry up."

The narrow-leaf variety is highly poisonous, and the cause of most of the poisoning in cattle. Its principal toxin is cardenolides-digitalis-like glycosides that cause the heart to act irregular causing cardiac arrest. All animals are susceptible to milkweed poisoning, and the toxin can also be ingested through milkweed in hay.